This application claims priority to Austrian application No. A 1796/98, filed Oct. 28, 1998, herein incorporated by reference.
This invention relates to two-component isocyanate crosslinkable systems based on aqueous dispersions of hydrophilic epoxy adducts. This invention also relates to binders containing the systems according to the present invention and a method for coating material using said binders.
Description of the Related Art
In unmodified form, epoxy resins, especially those based on bisphenol A which are commonly used commercially, are insoluble or of very low solubility in water. This means that an aqueous phase which is in equilibrium with the epoxy resin has a mass fraction of less than 1% of the epoxy resin. In principle, it is possible to obtain water-dilutable, cationically stabilized base resins by reacting epoxy resins with amines and then protonating the basic groups. By modifying the epoxy resin with nonionic hydrophilic groups or with anionic groups, it is likewise possible to achieve a limited solubility which is sufficient to give adequate stability to a dispersion of the particular modified epoxy resin in water. Such dispersions can be diluted with (additional) water. The modified epoxy resin can then be processed from the aqueous dispersion; after the water fraction has been removed by evaporation or after penetration into the substrate, the resin remains on the surface and, given a adequate composition of the disperse phase, forms a coalesced film which can be crosslinked chemically by adding appropriate curing agents.
In the case of the cationically modified epoxy resins, dilutability in water is achieved by neutralizing some or all of the basic groups of the epoxy-amine adduct with acids, such as formic acid, acetic acid or lactic acid, for example. In this case the level of basic amine groups in the epoxy-amine adduct (measured, for example, by way of the amine number; see below) and the degree of neutralization of said groups (i.e., the fraction of ionic groups) are critical for the extent of dilutability in water.
When formulating the batches care must be taken to ensure that the base resins have the level of amine groups required to ensure sufficient stability of the aqueous solution of the binder. Experience indicates a target amine number in the range from 30 to 100 mg/g.
A degree of neutralization of from 20 to 60% of the basic amine groups is usually sufficient for practical dilutability. The base resins are then diluted to the desired concentration using deionized water. The resin dispersions obtained in this way range from virtually clear to highly opaque and should have a readily manipulable viscosity in the range from about 100 to 4000 mPa.s at ambient temperature.
Cationically stabilized epoxy adducts of this kind are part of the prior art and have already been described on numerous occasions in the patent literature. In the field of cataphoretic electrodeposition coating, in particular, they are successfully employed in combination with blocked difunctional or oligofunctional isocyanates (see, for example, Austrian Patent Applications AT 1665/86, AT 1766/78, and German Patent Applications DE-A 30 41 700, DE-A 33 00 583, DE-A 33 11 513). If desired, they are subsequently processed further with crosslinking catalysts, pigments, fillers and other additives to form pigmented paints.
In contrast to electrodeposition coating applications, where the formulation of the base resins and thus the number and nature of the ionogenic groups strongly influence the electrophoretic applicability, the criteria governing, for example, the interior spray coating of drums are different.
One particular problem affecting the combination of water-diluted epoxy-amine adducts with unblocked isocyanates is the often very short processing time (pot life). Because of the basic medium, the unblocked isocyanate groups react with water even before baking, during application. This results in film defects due to gas bubbles (the reaction of isocyanate with water produces the corresponding amines and carbon dioxide) and reduced film crosslinking, since part of the crosslinking agent is consumed by the reaction with the water.
The epoxy-amine adducts which arc commonly employed in cataphoretic electrodeposition coating generally include a fraction of strongly basic amino groups with little steric hindrance, since such groups result in particularly favorable electrochemical deposition properties. For the reason given above, such adducts are poorly suited to combination with unblocked isocyanates.
It has surprisingly now been found that, under certain conditions, water-diluted epoxy adducts can be combined with unblocked isocyanates to form 2-component systems without entailing problems with film defects due to gas bubbles or inadequate film crosslinking.
The present invention therefore provides two-component systems based on aqueous dispersions of hydrophilic epoxy adducts Ah having hydroxyl groups as reactive groups and unblocked difunctional or polyfunctional isocyanates B. The hydrophilic epoxy adducts Ah can be selected from cationically stabilized hydrophilic epoxy adducts Ak, anionically stabilized hydrophilic epoxy adducts Aa and nonionically stabilized epoxy adducts An.
For this purpose it is necessary first of all, in the case of the cationically stabilized epoxy-amine adducts Ak, that only tertiary amino groups and not primary or secondary amino groups are present in the epoxy-amine adduct. The tertiary amino groups present must, furthermore, be sterically hindered. This is achieved by careful selection of the amines used and, in the course of synthesis, by direct attachment of the amino groups to epoxy groups without the formation of terminal amine groups in aliphatic side chains.
Owing to these conditions, amines such diethylaminopropylamine or dimethylaminopropylamine, for example, are unsuitable as raw materials since epoxy-amine adducts prepared from them contain terminal amino groups in aliphatic side chains. Amines which have proven particularly suitable, on the other hand, are those which carry either short-chain branched alkyl groups or hydroxyalkyl groups. When an adduct is formed with oxirane groups of the epoxy component, the result is then tertiary amine structures with strong steric hindrance which can nevertheless still be neutralized effectively with acids and so contribute to the water-solubility of the resins. In the case of the amines which carry hydroxyalkyl groups, additional OH groups are incorporated into the resin assembly and may act as crosslinking sites with, for example, isocyanate groups.
An important criterion for the epoxy-amine adducts Ak suitable for the invention is a sufficiently high number of OH groups, which are groups that are capable of crosslinking with isocyanates. OH numbers in the range from about 150 to about 400 mg/g have been found suitable.
The invention therefore provides two-component binders for the interior coating of drums, consisting of a cationically stabilized epoxy component Ak and a crosslinking agent B, wherein component Ak comprises exclusively those tertiary amino groups which are formed in the addition reaction from the amine component A2 and the epoxy component A1, and wherein the amine component A2 has a primary or secondary amino group and has no tertiary amino groups.
Compounds which can be employed as crosslinking agent B are in principle all those which undergo addition or condensation reactions with hydroxyl-containing compounds. Crosslinking agents B which have been found suitable in the context of the invention are primarily those which carry at least two unblocked isocyanate groups per molecule. Only using isocyanate crosslinking agents of this kind is it possible to produce chemically resistant coatings which cure rapidly.
Particularly suitable isocyanates are low molar mass isocyanates which are liquid at room temperature, having a viscosity of from about 50 to about 10,000 mPa.s. In these isocyanates, the isocyanate groups can be attached to aliphatic, aromatic or a mixture of aromatic and aliphatic structures. Use is made in particular of polyfunctional isocyanates or mixtures thereof having an average isocyanate functionality of from about 2 to about 5. In the context of the invention it is also possible to employ isocyanates which are solid at room temperature or are of relatively high viscosity, with inert solvents being added in order to lower the viscosity. Use is made in particular of solvents rich in aromatics, such as solvent naphtha, for example. Likewise suitable are the so-called paint isocyanates, which are obtainable by dimerization, trimerization or oligomerization of diisocyanates such as 1,6-diisocyanatohexane, 2,4- and 2,6-tolylene diisocyanate or isophorone diisocyanate to form the known biurets, uretdiones, isocyanurates or allophanates.
The epoxy component A1 is a commercially customary epoxy resin based on aliphatic or aromatic polyols, preferably diols, having a specific epoxide group content of from about 300 to about 11,500 mmol/kg. The specific epoxy group content SEG is defined as the ratio of the amount of substance of epoxide groups n(EP) in a sample and the mass mB of the sample (and is therefore the reciprocal of the so-called EV value or epoxide equivalent weight (EEW)); the customary unit of measurement is mmol/kg:
SEG=n(EP)/mB
Preference is given to epoxy resins based on bisphenol A and bisphenol F or mixtures thereof, having a SEG of from about 500 to about 10,000 mmol/kg. In addition, epoxy resins based on polypropylene glycol and having a SEG of from about 500 to about 5000 mmol/kg are also employed with preference.
The amine component A2 is preferably selected from secondary monoamines R1R2NH, R1 and R2 being selected independently of one another from the group consisting of linear, branched and cyclic alkyl radicals of 1 to 20 carbon atoms which if desired carry at least one primary hydroxyl group; preferably of 1 to 4 carbon atoms. A2 can further be selected from the group consisting of primary monoamines R3NH2, R3 being selected from linear, branched and cyclic alkyl groups of 2 to 20 carbon atoms, preferably 3 to 12 carbon atoms, with the proviso that R3 carries at least one primary hydroxyl group which is positioned xcex1 or xcex2 to the primary amino group. Examples of suitable amines are diisopropanolamine, diethanolamine, diisopropylamine, diisobutylamine, N-methylcyclohexyl-amine, monoiso-propanolamine and monoethanolamine.
The component Ak is obtainable by reacting an epoxy resin A1 having at least two epoxy groups per molecule with amines A2. This reaction is performed by first of all introducing the epoxy resin or a mixture of two or more epoxy resins and heating this initial charge to a reaction temperature of from about 80 to about 160xc2x0 C., preferably from about 100 to about 140xc2x0 C. By adding preferably aromatic polyols such as bisphenol A or bisphenol F, and using suitable catalysts, it is possible to reduce the specific epoxy group content in the manner of an advancement reaction. It is preferred to add inert solvents in order to lower the viscosity. After cooling to 60-100xc2x0 C., the amine component is added. The reaction is at an end when the specific epoxy group content is less than about 50 mmol/kg (EEW greater than 5 20,000 g/mol). After that, the solvent is removed by distillation, and the epoxy-amine adduct is at least partly neutralized with an aqueous acid, preferably an organic acid such as formic acid, lactic acid or citric acid, and is dispersed by addition of water, preferably in a plurality of portions, with thorough mixing.
DIN 53 176 defines the amine number as being the ratio of that mass mKOH of potassium hydroxide which consumes exactly the same amount of acid for neutralization as the sample under analysis to the mass mB of this sample (mass of the solid matter in the sample in the case of solutions or dispersions); its customary unit is xe2x80x9cmg/gxe2x80x9d.
DIN 53 240 defines the hydroxyl number as being the ratio of that mass mKOH of potassium hydroxide which has exactly the same number of hydroxyl groups as the sample under analysis to the mass mB of this sample (mass of the solid matter in the sample in the case of solutions or dispersions); its customary unit is xe2x80x9cmg/gxe2x80x9d.
A hydroxyl number of from about 150 to about 400 mg/g has proven suitable in order to obtain the required film properties such as crosslinking density, substrate adhesion, and flexibility.
The present invention additionally provides a two-component system comprising an anionically stabilized water-dilutable epoxy resin Aa and an isocyanate-functional crosslinker B.
One possibility of rendering epoxy resins dilutable in water with the aid of anionic or anionogenic groups is to modify the epoxy resin with acidic groups, especially with phosphoric acid groups or phosphonic acid groups. For this purpose the epoxy resins A1 can be reacted with polybasic acids A3 selected from phosphoric acid, alkyl-phosphonic acids R4PO3H2 of 1 to 18 carbon atoms, preferably 1 to 4 carbon atoms, in the alkyl group R4, aryl-phosphonic acids and hydroxyalkylphosphonic acids of preferably 1 to 4 carbon atoms in the alkyl group, and with the corresponding phosphonous acids R4PO2H2 in solution (with ketones, monoalcohols or mixtures of ketones and alcohols as solvent, for example); in this reaction, opening of the oxirane ring results in the formation of acidic xcex2-hydroxy esters Aa. About 1.0 mol of A3, preferably up to about 0.5 mol of A3, is used per mole of oxirane groups. The acid number of this epoxy-acid adduct Aa is preferably between about 15 and about 200 mg/g, with particular preference between about 20 and about 150 mg/g and, in particular, between about 25 and about 100 mg/g. To improve the solubility or dilutability in water, the adduct is subjected to at least partial neutralization, the degree of neutralization preferably being between about 10 and about 100% and, with particular preference, between about 20 and about 70%.
DIN 53 402 defines the acid number as being the ratio of that mass mKOH of potassium hydroxide which is required to neutralize the sample under analysis to the mass mB of this sample (mass of the solid matter in the sample in the case of solutions or dispersions); its customary unit is xe2x80x9cmg/gxe2x80x9d.
Instead of the epoxides A1 themselves it is also possible here to employ modified epoxides A16 which are obtainable by reacting the epoxides A1 with monocarboxylic acids A6 selected from saturated and unsaturated aliphatic carboxylic acids of 2 to 40 carbon atoms, preferably 8 to 18 carbon atoms, and aromatic mono-carboxylic acids whose aromatic ring can be substituted by alkyl, alkoxy, hydroxyl or halogen groups. In this case, the amounts of the reactants should be such that the molar amount of the carboxyl groups in A6 is less than or equal to about 40% of the molar amount of the epoxy groups in A1, preferably less than or equal to about 20% of this molar amount. This means that at least about 60% or, respectively, at least about 80% of the oxirane groups are unreacted in this reaction.
Anionically modified epoxy resins which can be employed for the invention can also be obtained by reacting mixtures of modified epoxy resins according to A16 and epoxy resins A1 with the polybasic acids according to A3 in solution.
A further class of epoxy resin-based binder which can be combined with unblocked isocyanates to form two-component systems is that of epoxy resins Aak having zwitterionic character. For this purpose, adducts A12 are prepared first of all from drying, oils A121, unsaturated fatty acids A122 or mixtures of these two components A121 and A122 with maleic anhydride A123. It is also possible to prepare individual adducts of the drying oils A121 and of the fatty acids A122 with maleic anhydride A123 and then to mix these adducts. The anhydride groups of these adducts are hydrolyzed by reaction with water or monoalcohols to give in each case two carboxylic acid groups or one carboxylic acid group and one ester group. Hydroxyl-containing epoxy-amine adducts A18, which are obtainable by reacting epoxy resins A1 with secondary amines A8, are subsequently condensed with these hydrolyzed adducts A12h. The mass fraction of the building blocks in the condensation product that originates from the hydrolyzed adducts A12h is up to about 40%, preferably between about 10 and about 35%. The acid number of the condensation product Aak is from about 10 to about 100 mg/g, preferably from about 15 to about 95 mg/g and, in particular, from about 20 to about 90 mg/g. The condensation step produces zwitterionic compounds Aak which are soluble or dispersible in water with the formation of anions by neutralization with amines or aqueous ammonia or of cations by neutralization with acids.
To prepare the epoxy-amine adducts A18 described above, it is also possible, instead of the epoxides A1, to employ those epoxy resins, described above as A16, which are obtainable beforehand by reacting the epoxides A1 with the above-described monocarboxylic acids A6. In this case the amounts of the reactants should be such that the molar amount of the carboxyl groups in A6 is less than or equal to about 40% of the molar amount of the epoxy groups in A1, preferably less than or equal to about 20% of this molar amount. This means that at least about 60% (or at least about 80%) of the oxirane groups are unreacted in this reaction. It is also possible to employ modified epoxy resins A17 which are obtainable by reacting epoxy resins A1 with polyhydroxy compounds A7, especially with aliphatic dihydroxy compounds selected from xcex1,xcfx89-diols of 2 to 8 carbon atoms and polyoxyalkylene glycols of 2 to 4 carbon atoms in the alkylene radical, with catalysis by, in particular, Lewis acids or adducts of Lewis acids with Lewis bases, examples being boron trifluoride, boron trifluoride etherates, tetrafluoro-boric acid, antimony pentafluoride, hexafluoroantimonic acid, etc. Here again it is the case that the amounts of reactants A1 and A7 should be chosen such that the amount of substance of the hydroxyl groups in A7 is less than or equal to about 40% of the amount of substance of the epoxy groups in A1, preferably less than or equal to about 20% of this amount of substance.
Particularly suitable amines A8 are secondary aliphatic amines having linear, branched or cyclic alkyl radicals of preferably 2 to 12 carbon atoms, which if desired also carry hydroxyl groups. Particularly suitable examples are diethanolamine and diisopropanolamine.
Examples of suitable drying oils A121 are linseed oil, wood oil, hemp oil, poppyseed oil, walnut oil, perilla oil, oiticica oil, safflower oil and fish oils and also dehydrated castor oils; particular preference is given to castor oils, safflower oil and linseed oil. It is particularly preferred to transesterify mixtures of drying oils by heating them together, alone or in the presence of catalysts, and then reacting the product with maleic anhydride as component A123. The resulting adduct is subsequently hydrolyzed by adding a sufficient amount of water or of a monoalcohol, which is accompanied by the liberation of carboxyl groups and, if appropriate, the formation of an ester group. Suitable unsaturated fatty acids A122 have 6 to 30 carbon atoms and at least one olefinic double bond; examples are palmitoleic acid, oleic acid, erucic acid, linoleic and linolenic acid, elaeostearic acid and arachidonic acid, and also the commercially customary mixtures of these.
In the context of the invention it is also possible to subject epoxy resins to anionic modification by first of all reacting hydroxy acids, mercapto acids or amino acids A4 having at least one isocyanate-reactive group selected from hydroxyl, amino and mercapto groups and at least one acid group selected preferably from carboxyl, sulfonic acid and phosphonic acid groups with an at least difunctional isocyanate A5 to give an intermediate A45 having at least one acid group and at least one isocyanate group. The acids employed preferably for this embodiment have acid groups which owing to steric hindrance do not themselves react, or react only very slowly, with isocyanate to form an amide and to liberate carbon dioxide. The intermediate A45 is subsequently reacted with a hydroxyl-containing epoxy resin A14, the proportions being chosen such that all of the isocyanate groups are consumed.
The hydroxyl-containing epoxy resins A14 are known and can be prepared, for example, by reacting diglycidyl ethers of diols with organic compounds having at least one oxirane-reactive hydroxyl group. The isocyanates A5 are likewise known and are selected from aliphatic, aromatic and mixed aromatic-aliphatic isocyanates having at least two isocyanate groups. Suitable examples are 2,4- and 2,6-tolylene diisocyanate, 1,6-diisocyanatohexane, isophorone diisocyanate and tetramethylxylylene diisocyanate. The acids A4 are, for example, aliphatic hydroxycarboxylic acids such as lactic acid, citric acid, tartaric acid and dimethylolpropionic acid, amino acids such as taurine, lysine and aspartic acid, or phosphonic acids such as hydroxymethanephosphonic acid.
The present invention additionally provides a two-component system comprising a nonionically stabilized water-dilutable epoxy resin An and an isocyanate-functional crosslinker B. Nonionically modified hydrophilic epoxy resins An are derived in particular from epoxy resins which include segments of oxyalkylene groups (preferably oxyethylene groups or mixtures thereof with oxypropylene groups). With these modified resins, of course, no neutralization is required in order to improve the dilutability with water. Particular preference is given to oxyalkylene segments which include a mass fraction of at least about 20%, preferably from about 40 to about 100% and, with particular preference, from about 50 to about 90% of oxyethylene groups.
The binders of the present invention are particularly suitable for the interior coating of containers which come into contact with aggressive chemicals.
Abbreviations Used:
Abbrev. Meaning Unit
EEW Epoxide equivalent weight, EV value g/mol
SEG Specific epoxy group content mmol/kg
OHN Hydroxyl number mg/g
AN Amine number mg/g
Nvc Nonvolatiles content (mass fraction of solids of g/ the resin solution or resin dispersion) (100 g)
MW Weight-average molar mass g/mol
EP 1 Diepoxy resin based on bisphenol A (SEG=5405 mmol/kg; EEW about 185 g/mol)
EP 2 Diepoxy resin based on bisphenol A (SEG=1080 mmol/kg; EEW about 925 g/mol)
EP 3 Diepoxy resin based on polypropylene glycol (SEG=2940 mmol/kg; EEW about 340 g/mol)
EP 4 Diepoxy resin based on bisphenol A (SEG=2000 mmol/kg; EEW about 500 g/mol)